The invention relates to a method for nitriding steel work pieces in a gaseous atmosphere containing atomic nitrogen at temperatures above 425.degree. C. and pressures above 0.2 MPa.
Nitride cases are generated in accordance with the current state of the art on work pieces of steel alloys in a salt bath by means of gas nitriding or by means of plasma nitriding. These nitride cases improve the corrosion resistance, the wear resistance and the oscillating resistance of the steels. They include a so-called "white layer" (i.e. the nitrogen rich layer), which is a few micrometers thick, over a nitrogen diffusion layer, which is generally produced by means of the above-mentioned gas nitriding or plasma nitriding methods at process times on the order of 100 hours. The diffusion layer includes nitrogen dissolved in the iron matrix, and it is located beneath the compound layer, wherein the compound layer includes the nitride layer and the white layer (i.e., the compound layer is an Fe.sub.x N.sub.x layer).
Gas nitriding has particularly received a great impetus in recent years. In conventional gas nitriding processes, the steel parts are heated in an atmosphere of nitrogen-releasing gases, preferably in an atmosphere of ammonia, in order to introduce nitrogen into the surface of the steel.
Nitrogen is produced during the dissociation of ammonia according to the following reaction EQU NH.sub.3 .fwdarw.N+3 H
The unstable nitrogen and hydrogen products react rapidly to form hydrogen gas (H.sub.2) and inert nitrogen (N.sub.2).
However, only nitrogen in the form of N atoms can be absorbed into the steel; therefore, only ammonia dissociating at the surface of the steel can supply the nitrogen for the case. The absorbed nitrogen diffuses into the steel and reacts to form precipitates of the nitrides of iron and any alloying elements. This precipitation creates compressive stresses which result in the case hardness.
Other alloying metals, such as aluminum, chromium, molybdenum, vanadium, or tungsten in solid solution, may be included with the steel to be nitrided. In fact, many plain carbon steels are known to produce a brittle case when nitrided. Stainless steel may be successfully nitrided.
Since no quenching is required after nitriding, before the nitriding process, the steels should be completely heat-treated so as to include the necessary properties in the steel base material. For example, 0.25-0.5 wt % carbon containing steel is quenched and tempered to the required core hardness prior to nitriding. In some alloys, the case hardness is directly proportional to the hardness of the underlying core steel.
Various metal case hardening processes, including nitriding, are described in Kirk-Othmer's Encyclopedia of Chemical Technology, Third Edition, Vol. 15, pages 313-323, which description is entirely incorporated herein by reference.
U.S. Pat. No. 2,779,697 teaches a method for nitriding steels in gaseous ammonia under pressure, which patent is entirely incorporated herein by reference.
In the method described in this patent, a pressure vessel is filled with a predetermined amount of ammonia and heated to temperatures between 425.degree. C. and 640.degree. C. (800.degree.-1200.degree. F.). This heating causes an ammonia pressure of a few bars to build up within the pressure vessel. Nitride cases of 20 to 40 .mu.m are obtained thereby within approximately 15 hours. The thickness of the nitride case is a function of the amount of ammonia, the pressure and the temperature. However, this method has not been able to gain general acceptance in practice.